A wide variety of IMDs have been developed in order to monitor patient conditions and deliver therapy to the patient. An IMD typically includes a hermetically sealed housing coupled to one or more leads that are surgically implanted inside a patient for sensing conditions or for administering therapy. The IMD may provide therapeutic stimulation to the patient or may deliver drugs or agents to the patient. Alternatively or additionally, the IMD may have sensing or monitoring capabilities. For example, the IMD may sense information within a patient and store the sensed information for subsequent analysis. In some cases, the sensed information may be used directly by the IMD to adjust or control the therapy that is delivered to the patent. Telemetry is used to communicate sensed information from the IMD to an external medical device so that analysis of the sensed information can be performed. Telemetry is further used to communicate information or instructions from external medical devices to the IMD. The IMD includes a telemetry module for performing such telemetry.
In order to perform telemetry communications with an external medical device, the telemetry module in the IMD can be programmed to perform so called “sniff” operations in which the telemetry module senses received signals to determine whether other devices are trying to communicate or are available to communicate with the IMD. The signals received by the telemetry module during these sniff operations are referred to as wake-up signals that direct the IMD to power on various components to communicate with the external medical device.